1. Field of the Invention
This invention relates to a composition of heat-sensitive electroresistive substances and a panel heater made therefrom, and more particularly to a composition of heat-sensitive electroresistive substances whose electric resistance varies abruptly with a positive characteristic at a certain temperature level, and also to a thermostatic panel heater using the foregoing composition.
2. Description of the Prior Art
Barium titanate ceramics doped with a rare earth element, Y, Bi, or Sb are known as heat-sensitive resistors. Although barium titanate in its pure state is an insulator, it turn to be a semiconductor as a result of doping.
Generally, resistivities of semiconductors show negative temperature characteristics, namely, the resistivity decreases with increasing the temperature of the semiconductor. In the case of doped barium titanate ceramics the resistivity show negative temperature coefficients in the lower temperature region, but positive temperature coefficients appear at a temperature which is slightly lower than the Curie temperature. Over the temperature range between the temperature described above and the temperature which is higher than the Curie temperature by .about.150.degree. C., the positive temperature characteristics are observed. Because of this positive temperature coefficient characteristics, doped barium titanate ceramics give a steady-state constant temperature when electric power is supplied. Therefore, doped barium titanate ceramics are called as PTCR (positive temperature coefficient resistor). Electrical resistivity for the heat sensitive substances of this invention exhibits positive temperature characteristics which is much stronger than that of doped barium titanate. Although the mechanism of the electrical conductivity in the heat sensitive substances of this invention could probably be different from that in doped barium titanate, the term "positive temperature charactertistic" of "PTC (positive temperature coefficient) effect" used hereafter in the same sense at that for doped barium titanate because both give the same electrical characteristics, namely, constant temperature under the supply of electricity.
Besides doped barium titanate ceramics as inorganic substances the system of carbon-paraffin-polyethylene has been known as an organic substance which has a strong positive temperature characteristics.
On the other hand, as an organic material having sufficiently strong positive characteristics, the ternary system of carbon-paraffin-polyethylene has been known. However, this composition is poor in compatibility with carbon, and has a problem in the mixing method as well as in change in characteristics with the passage of time.
When a heat-sensitive electroresistive composite having a positive characteristic is raised in temperature from low range, the value of resistance increases precipitously when the temperature reaches a certain point. Also, in the contrary, when the temperature of the composite is lowered from the higher temperature range above the aforesaid certain point, the value of resistance decreases sharply at the certain point mentioned above. Now, when the composite provided by this invention is heated by supplying electricity, while the amperage of the electric current is high during the initial period, after a while, upon reaching a certain temperature the current becomes weak because the value of resistance increases suddenly. Thereafter, when the temperature of the composite is lowered by cooling it the resistance value decreases. As the result, the electric current becomes high in amperage again, and the temperature returns to the certain level. By utilizing the property described above, this composite can be used as a temperature sensor, temperature fuse, and thermostatic heating element.
Organic compounds with melting point around room temperature .+-.50.degree. C., with the desirable properties, such as high thermal stability, low toxicity, and also are a good insulator of electricity, are found in large number. Such organic compounds are, for example, paraffins, polyalkylene glycols, higher alkyl ethers, higher alkyl esters, higher fatty acids, and higher alcohols.
Also, these organic compouhds are melted when heated from outside up to the degree above the melting point, and the heat is stored as latent heat of fusion, in those compounds. Accordingly, they are known as a regenerative media. By using systems without requiring additional installation of a temperature sensor and a thermal fuse separately inside of the heating unit.
In other words, the heat-sensitive electroresistive composite provided by this invention has the property that its temperature does not go up beyond a certain level as an inherent property of a resistor.
Consequently, the heating element using the foregoing composite is completely free of malfunction, and it is also extremely dependable and safe. Furthermore, in view of the energy consumption, since its temperature does not exceed a specified point, it suffers no energy loss. Thus, in addition to the above, it has an outstanding merit in terms of economy.
The present invention is intended, therefore, to provide a heat-sensitive electroresistive composite having desirable thermostatic properties including a positive characteristic. Furthermore, the inventor of this invention has designed a way to make heating elements, particularly sheet form (panel) heating type units, by sealing the composites together with electrodes in insulators. In this manner, the inventor has developed heating units which are quite suitable as the base materials for floor heating systems of buildings, heating carpets, heating mats for agricultural and stock farming applications, such as brooder, breeder, seedling culture, as well as for other uses.
This invention was achieved based on the following findings. That is, the inventor found that, among organic compounds, polyethylene glycol is remarkably high in compatibility regenerative media, the heat generated by the electric heater receiving the electricity from a generator using an irregular natural energy source, such as wind power (areogenerator), tidal power (tidal power plant), and solar energy (solar power plate), can be stored. However, the regenerative medium itself is a good insulator of electricity and it is impossible to energize it with electricity directly for heating. Therefore, it is necessary to heat it by using an electric heater, and also a thermostat or a thermoprotector must be provied for temperature control. This inevitably increases the cost for equipment.